Optical filters are devices that selectively transmit light of certain wavelengths while attenuating light outside of these wavelengths. Such filters most typically comprise plane glass or plastic devices, inserted in an optical path, which are either dyed in the bulk or utilize interference coatings. Such devices can be completely described by their frequency response, which specifies how the magnitude and phase of each frequency component of an incoming signal is modified by the filter.
Filters that pass long wavelengths only are generally referred to as longpass filters, filters that pass short wavelengths only are generally referred to as shortpass filters, and filters that pass a band of wavelengths, attenuating both longer and shorter wavelengths, are generally referred to as bandpass filters. In bandpass filters, the “passband”, or wavelengths which are allowed pass through the filter, may be narrow or wide and the transition or cutoff between maximal and minimal transmission can be sharp or gradual.
Optical filters can be classified, generally, into two primary categories: absorptive filters and interference, or dichroic, filters. Dichroic filters may also be referred to as “reflective” or “thin film” filters. Absorptive filters selectively absorb certain wavelengths passing therethrough and reradiate the electromagnetic energy absorbed in a different form, such as thermal energy. Absorptive filters are typically the less expensive of the two, but are not generally suitable for precise scientific work.
Dichroic filters use the principle of interference and most typically function by reflecting the unwanted portion of the light and transmitting the remainder using thin, transparent optical substrates coated with a series of thin dielectric layers deposited on the surfaces thereof, using various techniques known to those skilled in the art. Opposite the dielectric layers, there is typically an anti-reflection coating, or other coatings to provide specified transmission characteristics. These layers form a sequential series of reflective cavities that resonate with the desired wavelengths; other wavelengths destructively cancel or reflect as the peaks and troughs of the waves overlap.
Since the exact wavelength range of a dichroic filter can be precisely controlled by altering the thickness and sequence of the coatings, they are particularly suited for precise scientific work. They are usually, however, much more expensive and delicate than available alternatives, such as the previously-mentioned absorption filters. Filters of this type are also commonly used in devices such as the dichroic prism of a camera to separate a beam of light into different colored components.
Another optical device well suited for precision scientific work is a Fabry-Pérot interferometer. Such an interferometer uses two mirrors to establish a resonating cavity: only those wavelengths that are a multiple of the cavity's resonance frequency are passed.
Etalons are another variation: transparent cubes or fibers whose polished ends form mirrors tuned to resonate with specific wavelengths. These are often used to separate channels in telecommunications networks that use wavelength division multiplexing, as is used on long-haul optical fiber networks.
Because of limitations inherent in this technology, the narrowest band-pass filters that can be achieved using current state of the art filters have spectral band-pass values in the range of 50 to 100 nm, for Long-Wavelength InfraRed (LWIR) operational wavelengths (8-12 microns). For some applications, however, it is beneficial and desirable to have an optical band-pass that is significantly narrower (smaller). Filters capable of filtering as low as 5 to 15 nm of optical bandwidth would be particularly desirable. Although such band-pass values can be achieved in filters operating in the visible wavelengths, these values are unachievable in the LWIR using current state of the art filters.
FIG. 1 depicts the Dewar configuration of a conventional LWIR sensor in the region surrounding the focal plane, the plane through the focus perpendicular to the axis of a mirror or lens. The device preceding the focal plane is the optical band-pass filter, which determines the spectral width of the optical band-pass filter preceding the focal plane array. Typically, because of the limitations of dielectric filter technology, the spectral width preceding the focal plane array is limited to 100 nm, but, for some applications, it may beneficial if this value was considerably less.
What is needed, therefore, are techniques for decreasing the lower limit of band-pass filters operating in LWIR and similar wavelengths while narrowing the filtration range.